rdbms: Add a deprecationLogger callback

rdbms: Add a deprecationLogger callback

Much like the existing errorLogger, but for logging deprecation
warnings.

The default in the RDBMS layer is to call trigger_error() with
E_USER_DEPRECATED. The default in MediaWiki (via MWLBFactory) is to log
to the 'deprecated' log group, much like wfDeprecated() does, although
unfortunately we can't effectively use that directly since we have no
idea of a proper $callerOffset to pass.

Change-Id: Id13625e249516e84d72b6310953bb338a90976da